1. Field of the Invention
The present invention relates to a method of manufacturing implant parts made of highly cross-linked polyethylene, in the following termed UHMWPE (ultra-high molecular weight polyethylene), as well as to parts of an implant for application in human medicine. Often such parts are used as a covering layer and/or as a material for slide bearings, provided in order to optimize physicomechanical properties.
2. Description of the Related Art
In the past “normal” UHMWPE was used for this purpose, but it proved to have undesired physiologically relevant side effects ascribable to inadequacy of that form of UHMWPE with respect to homogeneity and resistance to abrasion. For example, when “normal” UHMWPE is used as a sliding bearing in artificial hip joints, abrasion produces microscopically small particles that become distributed in the surrounding tissue and can produce tissue necrosis and osteolysis. The body reacts with inflammations and tissue alterations. Hence a long-term employment of such implant parts—although highly desirable in itself—is impossible. Repeated operations are needed.
Subsequently, therefore, the material used for such implant parts was highly cross-linked polyethylene that had been produced from conventional UHMWPE by additional cross-linking steps. This cross-linkage was customarily achieved by way of intermediate stages involving radicals that are produced either by means of ionizing radiation or, as an alternative, by chemical means.
An essential prerequisite for the application within an organism of the highly cross-linked polyethylene thus produced, however, is that the radicals appear merely in a transitional state and are completely eliminated by subsequent reactions; otherwise, as is generally known, the radicals could bring about destruction of tissue, for instance by acting as carcinogens.
Complete elimination of the radicals, however, has proved problematic, because their stability and hence their lifespan is “positively” influenced by various kinetic and/or thermodynamic effects. In the matrix of interest here, namely cross-linked polyethylene, the alkyl groups have a stabilizing action and there is also a steric intervention by substituents with a blocking action owing to strong cross-linkage, or reduced mobility resulting from the strong cross-linkage, so that some radicals hardly have any opportunity for recombination and thus remain more or less isolated in the matrix—although in principle, later reactions can allow diffusion of these radicals.
The ultimate result is that incomplete elimination of the radicals impairs the properties of the implant part, for instance by subsequent oxidation, and hence leads to the problems cited above with respect to the homogeneity and resistance to abrasion of the polyethylene of which that part is made.
This problem has long been acknowledged, and attempts have been made to solve it in various ways.
These approaches are based substantially on tempering the workpiece at an elevated temperature, in the region of its melting point, in order to increase the mobility of the radicals remaining in the workpiece and enable their recombination.
Such a procedure is described, for example, in the patent U.S. Pat. No. 6,017,975, which proposes heating of the irradiated matrix to a temperature of about 150° C. until substantially all the free radicals are recombined. What is not considered here, however, is that whereas heating the matrix does make it semifluid on account of its high degree of cross-linkage, heating alone cannot eliminate the steric interference caused by the cross-linkage, so that although the mobility of the radicals is somewhat increased, a reaction of all the radicals, especially the sterically hampered ones, is practically impossible within a reasonable period of time.
In the end result this means that the time required for recombination of all free radicals, in particular also because the structure of the matrix is not constant from one case to another, is unpredictable and can be almost arbitrarily long.
A similar approach has been taken in the patents U.S. Pat. No. 5,414,049 and EP 0 722 973 A1. There the irradiated matrix is likewise tempered; in the US document the objective is likewise to eliminate free radicals, whereas in the EP document this is merely a concomitant reaction, the main goal being that the tempering should prevent the possibility of later shrinkage of the matrix. The problems associated with tempering alone, as described above, also apply to these last two patent documents.
A completely different approach is that of U.S. Pat. No. 5,577,368, the objective of which is to use an external hydrogen partial pressure during irradiation in order to reduce the number of remaining free radicals. Here the disadvantage is that the cross-linkage of the polythene is diminished in comparison to the methods cited above, because breaks in the chain binding can be filled in by saturation with hydrogen, without the formation of a carbon-carbon bond.
The patent WO 97/29793 A1 describes prostheses made of irradiated UHMWPE and other objects consisting of this material, in each case without free radicals, as well as several methods of manufacturing such UHMWPE, in which the irradiation is carried out exclusively with electron beams.
The patent EP 1 072 274 A1 describes a method of manufacturing sliding-bearing layers, in which a polymeric material, preferably UHMWPE, is exposed to irradiation with gamma rays in order to bring about a desired degree of cross-linkage. This material is then cross-linked at a temperature above the melting point to increase its ability to resist oxidative decomposition. Subsequently the product is shaped as desired and again irradiated with gamma rays to sterilize it.
The patent EP 1 072 275 discloses a method of manufacturing sliding-bearing layers made of UHMWPE for orthopedic implants, in which a cross-linkage of the polymers is brought about by irradiation with gamma rays, X-rays, ultraviolet rays, or neutron, proton or electron beams, in order to cross-link at least part of the UHMWPE.
Finally, the patent EP 1 086 709 A1 discloses a method of improving the sliding-bearing properties of molded parts made of UHMWPE for orthopedic implants, in which the UHMWPE is irradiated with electron beams and thus a reduction or elimination of the free radicals and cross-linkage of the product are achieved.